Accurate inspiratory minute volume (IMV) is required for us Army fixed and rotary wing aircraft oxygen system design. This initial study evaluated oxygen usage rates of US Army aircrew conducting operational missions at altitudes of 19,000 to 25,000 feet flying RU-21H twin turbopropeller driven unpressurized aircraft. Inspiratory minute volume (IMV) was calculated from the crew dilutor demand oxygen regulator pressure gauge and timed mission profiles. The IMV results were consistent with consideration of the limited accuracy of the pressure gauge through 56 flights with 112 pilot and copilot crewmembers. The average IMV was 8.09 + or - 2.14 Standard Deviation (SD), liters per minute (LPM) at normal temperature (70 F), pressure (760 mmHg) and dry NTPD). The range of IMV was 4.47 to 13.25 LPM NTPD per crewman.